Streptococcus pneumoniae is a leading cause of bacterial pneumonia, meningitis, and sepsis in children. Recent estimates of child deaths caused by S. pneumoniae range from 0.7-1.0 million every year worldwide. In 2000, about 14.5 million episodes of serious pneumococcal disease (uncertainty range 11.1-18.0 million) were estimated to occur. Pneumococcal disease caused about 0.826 million deaths (0.58-0.926 M) in children aged 1-59 months, of which 0.091 million (0.063-0.1 M) were in HIV-positive and 0.735 million (0.51-0.82 M) in HIV-negative children.
The multivalent pneumococcal polysaccharide vaccines that have been licensed for many years have proved valuable in preventing pneumococcal disease in adults, particularly, the elderly and those at high-risk. However, infants and young children respond poorly to unconjugated pneumococcal polysaccharides. The pneumococcal conjugate vaccine, Prevnar®, containing the 7 most frequently isolated serotypes (4, 6B, 9V, 14, 18C, 19F and 23F) causing invasive pneumococcal disease in young children and infants at the time, was first licensed in the United States in February 2000.
Further Prevnar™ 13 (Wyeth) is an approved vaccine that contains conjugates of polysaccharides from serotypes 6A, 6B, 19A, 19F in addition to 1, 3, 4, 5, 7F, 9V, 14, 18C and 23F. Synflorix™ (GSK) is another approved vaccine that provides protection against 1, 4, 5, 6B, 7F, 9V, 14, 18C, 19F, and 23F as well as cross protection against 19A & 6A.
Vaccine formulations must generally be stable and be of uniform consistency to accommodate the need for a long shelf-life and the use of multiple dose containers. Vaccines based on proteins, including polysaccharide-protein conjugates, are subject to protein aggregation and precipitation which can result in an effective lower total concentration of the vaccine due to the unavailability of the precipitated protein product. Polysaccharide-protein conjugate vaccines, in particular, appear to have a stronger tendency to aggregate than the carrier protein alone (See Berti et al, 2004, Biophys J 86:3-9). The choice of formulation for a polysaccharide-protein conjugate vaccine can greatly affect protein aggregation. See Ho et al., 2001, Vaccine 19:716-725.
Inspite of several existing multivalent pneumococcal polysaccharide-protein conjugate vaccine compositions being developed worldwide, there is an ongoing need in the art for vaccine formulations that provide high adsorption of individual conjugates and free from the aggregation/precipitation of immunogenic compositions having polysaccharide-protein conjugates.
The adjuvants traditionally used in such multivalent pneumococcal vaccines have been aluminium salts such as aluminium hydroxide and aluminium phosphate. Many other experimental adjuvants are known, however adsorption to aluminium salts remains the most common vaccine adjuvant formulation. Although their use is widespread, aluminium salts may not always be compatible with particular antigens thereby resulting in significant variations with respect to percent adsorption of polysaccharide-protein conjugate on alum or the antigenicity.
Immunological properties and stability of conjugate vaccine candidate adsorbed on aluminium adjuvants depends on various parameters like i) antigenicity of each antigen, ii) type of carrier protein used for conjugation, and iii) the type of adjuvant used. Most importantly, the extent of adsorption of antigen on the adjuvant has been previously reported to be one of the key parameters to demonstrate the lot to lot consistency of the formulation process and its possible impact on the efficacy of the vaccine product. Further, percent adsorption of polysaccharide-protein conjugates may drop further on storage of the formulation or under the adverse situations like temperature excursions. Refer 54th meeting of the WHO Expert Committee on Biological Standardization, Recommendations for the production & control of Pneumococcal conjugate vaccines, 17-21 Nov. 2003; Carl E. Frasch, Session IV: Conjugate Vaccines; Vaccine Technology II; Portugal. 2008.
As per European regulatory agency (EMEA) guidelines for pneumococcal polysaccharide-protein conjugates, completeness of adsorption (% unbound conjugate) should be considered as a crucial quality control parameter along with alum content, sterility, identity and free polysaccharide content. Refer Assessment Report for Synflorix 2009, Procedure No. EMEA/H/C/000973. WHO recommends maximizing adsorption for alum-precipitated antigens (e.g. diphtheria and tetanus toxoid) wherein at least 80% of the antigens in these vaccines be adsorbed.
During manufacturing of polysaccharide-protein conjugate, formulate can comprise of aggregates of polysaccharide-polysaccharide type, protein-protein type or polysaccharide-protein type. Such aggregations are also observed in the finished product leading to rejection of 4% to 10% of filled vials of polysaccharide-protein conjugate(s) vaccine, thereby affecting the stability and efficacy of the conjugate vaccine.
Given the above discussed limitations with respect to the aggregation and stability of the polysaccharide and its conjugate, there remains a distinct need for reducing aggregation and stabilizing said polysaccharide across downstream processing to the final formulation stage of Pneumococcal Conjugate Vaccine Manufacturing.